Electric utility grids have traditionally been hierarchical and centralized, meaning that the energy flow is optimized for the transfer of electric energy from large generation stations downstream to consumers at the end of the line. In other words, the grid structure is designed to transport energy in one direction, namely, downstream from producers to consumers. Moreover, an electric utility infrastructure itself is optimized, or designed, at all points to tolerate a certain maximum load, while operating in this hierarchical manner.
The grid structure, in a simplified sense, may be compared to a tree, where the trunk transmits electric energy, for example, from a generating station, analogous to the roots of the tree, at high voltage through limbs and branches, which gradually divide and split to deliver the electric energy to consumers at lower voltages. In practice, unlike in an actual tree, a certain amount of branch redundancy is built into the grid, so that more than one transmission route may be available between a source, such as a generating station, and ultimate users, in case some branches become unavailable or are out of service for some reason.
In an electric utility grid, the trunk and branches are more commonly referred to as lines, which are linked to one another at nodes. Nodes allow switching functions between several connections, like network switches, but they must allow for arcs that might form during a switching operation to subside, and the grid to stabilize, before performing a subsequent switching operation. Each line and node is designed and optimized for power flow down the tree. Both lines and nodes have rated capacities, which are the maximum loads that each can carry. These ratings are not necessarily fixed, but can vary depending upon the age of the equipment and its physical condition, as well as upon such weather conditions as ice, wind, and temperature. The electric utility infrastructure, as a whole, is optimized to tolerate a certain maximum load at different nodes of the grid under the assumption of a hierarchic energy flow.
Electric utilities always need to balance supply and demand, as any mismatch between supply and demand may create an imbalance in the grid. Although there are a few technologies for storing relatively small amounts of electric energy, such as magnetic storage, dynamic storage, fluid storage, and batteries, there is as yet no technology for temporarily storing large amounts of electric energy to maintain the balance if supply exceeds demand. As a consequence, the supply being made available must match demand to a large extent. It will be appreciated that some amount of prediction is involved to anticipate the demand in coming hours and days, so that the proper supply may be generated and made available.
Traditionally, electricity has been produced at large generating stations from energy derived from the burning of coal or natural gas, nuclear fission, or hydroelectric sources. Historically less important contributions, from the viewpoint of percentages, have been derived from such renewable, or “green”, sources as geothermal, wind, and solar energy sources. In recent years, consumers themselves, with the purchase of the appropriate equipment, have been able to produce their own electricity using wind and solar sources, and are able to send any unneeded excess out into the grid. As a consequence, with technologies that have recently become available, consumers of electricity can now become both producers and consumers, or “prosumers”, of electricity, turning the traditional utility grid into a network having sources scattered throughout.
The prosumers, of course, provide the network with additional sources of electric energy. However, wind and solar sources are largely weather-dependent and are impacted by storms, as well as by variations due to local geography, season and climate. Wind power is clearly driven by the characteristics of the local winds, although turbine performance will be affected by temperature and precipitation, as well as by the three-dimensional variation in the wind speed and direction. Solar power clearly only applies during the daylight hours, but it will vary depending on cloud cover, temperature, precipitation, and wind, as well as on aerosol content, as indicated by atmospheric turbidity. Hence, these renewable sources introduce some uncertainty into the network because they are intermittent and variable. Most utility grids, designed with a hierarchical structure with a source at the top controlled by the producers, are not designed to operate with this uncertainty and intermittency. As a consequence, the grid may get overloaded if several prosumers start to send electricity out into the network at the same time. In turn, grid instability, with an increased stress level on the infrastructure, a lower quality of delivered power, and rolling blackouts, may result. The challenge, then, is to enable the right balance between traditional base generation and intermittent sources, while maintaining economic grid stability, and avoiding sudden over- or under-loading.
The present invention provides a way to address this uncertainty for grids having consumers who are also producers of electricity.